These days a great number of high rise buildings have been constructed. They are mostly designed with a sealed space. Hence air conditioning system is very important for those buildings.
An A/C system generally equipped with a negative pressure (draw through type) air handling unit or positive pressure (blow through type) air handling unit, the drain pan outlet should be on the downstream side of the coil. A conventional condensate drain trap.
Please refer to FIG. 1, an air handling unit 1a which is mounted onto a foundation base 3a and coupled with a trap 2a on one side. The trap 2a generally is a U-shape siphon and a leg of the trap 2a must be at least long enough so the column of water in the trap 2a will overcome the fan pressure in the coil plenum, if the leg of the trap 2a is too shot on a negative-pressure drain, water will not flow out the drain but will accumulate and eventually overflow the drain pan. On a positive-pressure drain, insufficient trap leg length will allow air to flow out of the drain continuously.
Although the conventional structure set forth above can be function as a condensate drain trap but still have problems, notably:                1. The leg of the trap 2a must be long enough, and the height of the foundation base 3a has to be carefully set. Otherwise drain quality will be affected.        2. The trap outlet pipe can't lay on the floor, elevation difference between the drain pipe and the ground surface is not convenience for drain line installation.        3. The U-shape trap tends to accumulate dirt that is difficult to inspect and clean.        
To remedy the foregoing disadvantages, another kind of condensate drain trap for positive or negative pressure air handling unit has been developed as shown in FIG. 2. It has a water drain valve 1 with two sides connecting to a water inlet 11 and a water outlet 12. The water inlet 11 and the water outlet 12 are joined at a juncture which houses an elastic element 13 to press a retaining element 14. Water may push the retaining element 14 and compress the elastic element 13 to open the water outlet 12 to drain water. Such a structure provides a condensate drain function for positive or negative pressure air handling unit, and makes cleaning easier. But the water drain outlet still is spaced from the ground surface at a great distance, and is inconvenient for drain line installation. As shown in the drawing, it needs a plurality of bolts 15 to fasten and anchor the retaining element 14 and the elastic element 13. Water drain is controlled by the elastic element 13. Hence design of the elastic element 13 has to be precise. Moreover, elastic fatigue takes place as the elastic element 13 is subject to compression frequently. As a result, the retaining element 14 cannot block the water outlet 12 and external air will flow back into the air handling unit.